This invention relates to the selective removal of acid gases such as carbon dioxide, hydrogen sulfide, and sulfur dioxide, other sulfur-containing compounds such as carbonyl sulfide, and other relatively high boiling point gases, generally regarded as contaminants, from gas mixtures also containing lower boiling point components such as hydrogen, carbon monoxide, methane, and other light molecules such as nitrogen, some or all of which may be of primary value. The invention has particular application to the selective removal of acid gases and other sulfur-containing gases from, for example, the gaseous products of coal gasification, so as to produce a fuel gas end product of enhanced value and utility. The invention is particularly useful also in the selective removal of similar contaminants from the products of combustion of methane or other carbon-containing fuels to produce hydrogen and nitrogen in the manufacture of ammonia. A simplified form of the invention has particular application to the removal of sulfur-containing compounds and also suspended particulate matter from stack or flue gases. Various other uses for the invention will be recognized by those skilled in the art.
This invention also relates to a novel process for crystallization of a component of a fluid mixture by freezing. This crystallization process is useful as a part of the main gas separation process referred to above and is also adapted for more general use to separate a material that is crystallizable by freezing from other material. For example, this novel crystallization process may be used to separate carbon dioxide from hydrogen sulfide in the main gas separation process referred to above to provide substantially pure carbon dioxide and relatively concentrated hydrogen sulfide. In addition, by way of further illustration, it may be used to separate the isomers of xylene, ethane from carbon dioxide, or sulfur hexafluoride from hydrogen sulfide and/or light hydrocarbons. This novel crystallization process may be performed as a batch process or as a continuous process.
Many methods have been developed for effecting the selective separation of acid gases from other gases of primary value. Usually, a chemical or physical absorbent for the acid gases to be separated is contacted by the gas mixture being treated, the absorbent and absorbed acid gases are separated, and the absorbent is regenerated and recycled. The Benfield hot carbonate process is a typical example of processes using chemical absorption. See Pipeline and Gas Journal, Oct. 19, 1972, p. 58. The Rectisol refrigerated methanol process is a typical example of processes using physical absorption. See Industrial and Engineering Chemistry, July 1970, pp. 39-43.
Particularly in the case of chemical absorption processes, but also to a substantial degree in the case of most physical absorption processes, there are substantial, inherent irreversibilities in both the absorption and regeneration steps. These irreversibilities necessitate substantial energy inputs to the processes. For example, in the Benfield hot carbonate process, substantial amounts of steam are needed to regenerate the alkaline carbonate solution employed as the absorbent. And, in the Rectisol refrigerated methanol process, substantial amounts of steam and refrigeration are needed to regenerate the methanol absorbent. Thus, an undesirable characteristic of prior acid gas removal processes is their inherent, substantial energy consumption in regenerating the absorbent.
In many of the prior gas separation processes, the absorbent streams gradually accumulate impurities that have no value and would cause objectionable pollution if discharged into the environment. In those cases, additional capital and operating costs must be incurred for processing contaminated absorbent bleed or slip streams. Many of the prior gas separation processes also inherently involve substantial losses of absorbents due to minor poisoning reactions, leaks, thermal degradation, evaporation into the purified gases, and the slow accumulation therein of tars and other heavy materials. Make-up for these absorbent losses represents a continuous operating cost.
Another undesirable characteristic of many prior acid gas removal processes is that they require high capital and operating costs to recover the separated hydrogen sulfide and other sulfur-containing gases in a sufficiently high concentration for economical processing in a Claus plant to reduce them to elemental sulfur and non-polluting wastes. See Hydrocarbon Processing, April 1971, p. 112.
Another undesirable characteristic inherent in some of the prior gas separation processes is that they require the use of absorbent solutions that are corrosive or become corrosive in use. This requires periodic replacement of equipment or the use of expensive corrosion-resistant materials or expensive corrosion inhibiting chemicals.
Another disadvantage of many prior gas separation processes is that much of the available pressure and thermal energy of the purified gas stream and of the separated gases and reagents is not recovered. More reversible processes could recover and utilize such potentially available energy.
Another disadvantage of many prior gas separation processes is that they use relatively viscous absorbents or reagents, which decrease absorber stage efficiency and consume significant amounts of energy for pumping.
Another disadvantage of most prior gas absorption processes is that expensive heat exchangers or excessive absorbent flows are necessary to remove heat of absorption when large amounts of gas are absorbed.
Still another undesirable characteristic inherent in some of the prior acid gas removal processes is their inability to remove trace impurities that are undesirable in the purified product gases. Typical trace materials, depending on the sources of the gases to be purified, may include metal carbonyls and sulfur-containing molecules (other than hydrogen sulfide), such as carbonyl sulfide, carbon disulfide, mercaptans, and the like, relatively high boiling nitrogen-containing compounds, including ammonia and hydrogen cyanide, and relatively high boiling hydrocarbons (as hereinafter defined). The inability to remove such trace impurities results in end product gases of lesser value or reduced utility. Of the various trace impurities encountered in acid gas removal processes, carbonyl sulfide is particularly objectionable and generally must be removed if present in a gas stream being treated. Some prior acid gas removal processes are incapable of doing so without substantially increasing absorbent flows, which requires large additional capital and operating costs.
To the extent that prior art crystallization techniques are employed in such prior acid gas removal processes, there are additional disadvantages associated with known crystallization processes which are briefly described below.
Crystallization is potentially an attractive method of separating the components of a system or mixtures of materials, since impurity concentrations in the crystals are typically one tenth, and one-hundredth, or even a lesser fraction of the impurity concentration in the mother liquor. Thus, a few recrystallizations can often produce a highly purified product. However, crystallization processes used to date have certain practical shortcomings which limit their more widespread use. In many conventional crystallization processes, the crystals are formed by freezing on heat exchange surfaces which are scraped (mechanically) to remove the crystals. The crystals are then conveyed to a washing zone and separated from the liquid. Melting is accomplished by supplying heat through heat exchange surfaces. The heat exchange surfaces needed for freezing and melting and the mechanical scraping and conveying equipment are costly and troublesome.
In some improved crystallization processes, certain of the foregoing disadvantages are overcome by using an in situ refrigerant to form the crystals. Evaporation of refrigerant from the liquid mixture produces crystals directly. This type of process has the advantage of eliminating heat exchange surfaces for cooling, but the other costly aspects of crystallization noted above are usually retained.
Another disadvantage of most prior art crystallization processes is that they are not particularly energy-efficient. For example, temperature driving forces for heat exchange are often several tens of degrees. Further, the removal of a solvent such as water from material such as salts and sugar requires substantial energy to be added to the system.
The use of crystallization processes at triple point conditions for desalination of sea water is described in Rudd, Powers, & Sirola, Process Synthesis, pp. 259-280 (1973); Chemical Engineering, May 7, 1979, pp. 72-82 and Scientific American, December 1962, pp. 41-47. These prior art processes include direct contact heat exchange systems with and without the use of a secondary refrigerant. In a single stage system, the use of evaporative cooling to form a solid phase and the melting of the solid phase by direct contact with a compressed evaporation vapor are also described. It is believed that these prior art systems were not found to be entirely satisfactory due in part to their inability to provide economically high production rates. Specifically, the production rates were limited by the contamination of the evaporation vapor with salt due to its entrainment during the extremely agitated conditions of evaporation (or more descriptively, boiling) at high production rates. The contamination problems were resolved through the use of an increased number of lower production rate vessels in the process, but this solution resulted in economically undesirable increases in the capital costs of the process and of substantial abandonment of the process.
By way of summary, all of the prior acid gas removal processes have had a number of serious disadvantages involving troublesome problems and/or excessive capital costs and/or operating costs. Further, the use of known crystallization processes in such acid gas removal processes would give rise to similar disadvantages particularly associated with prior crystallization processes.